Power amplifier performance for emerging 5G mm-wave systems poses significant challenges for output power, efficiency and linearity. Efficiency in backoff is a key concern, given the peak-to-average power ratio of order 6-9dB for 5G signals. As a result, considerable attention has been given to composite amplifiers featuring backoff efficiency enhancement, particularly Doherty amplifiers. Adaptive bias circuits have been previously developed for use with power amplifiers at low microwave frequencies (for example, 1-2GHz as applied in 2G, 3G and 4G cellular networks). Direct application of these techniques is not straightforward at higher frequencies, such as 28GHz as used for 5G wireless communications, because the transistors have less gain at the high frequencies.